Apparatus for detecting arcing faults and ground faults in multiwire branch electric power circuits

ABSTRACT

A multiwire branch circuit including two line conductors and a grounded, common neutral conductor is protected by a two pole circuit breaker connected to independently interrupt overcurrent conditions in the two ungrounded line conductors. Three separate protection circuits provide arc fault protection for each of the ungrounded line conductors and ground fault protection for all three conductors. The arc fault detectors use the bimetal of the thermal-magnetic trip device for the associated line conductor for current detection, and therefore, are individually referenced to the associated line voltage. Hence, the outputs of the arc fault detectors are electrically isolated from each other and from the output of the ground fault detector, but operate a common trip circuit to simultaneously open both poles of the two pole circuit breaker. The arc fault detectors have separate isolated power supplies. The ground fault detector is powered by a supply which is energized if either of the ungrounded line conductors is energized.

RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/939,263 filed on Sept. 29, 1997, now U.S. Pat. No. 5,889,643issued Mar. 30, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to apparatus providing protection in multiwirebranch circuits of electric power distribution systems, and inparticular, to circuit breakers providing protection from arcing faultsand ground faults in such circuits.

2. Background Information

Branch circuits in electric power distribution systems often share acommon neutral conductor to reduce the wiring required. Such multiwirebranch circuits are often referred to as "home runs." Home runs are onlypermitted under certain conditions. Generally, they are permitted aslong as the two line conductors are energized by separate phases or by acenter tapped single phase to avoid overloading the neutral conductor,and as long as all ungrounded conductors are disconnected simultaneouslyat the panel board where the branch circuits originate. Thissimultaneous disconnection of the ungrounded conductors can beaccomplished with a two pole disconnect, two single pole circuitbreakers with a handle tie, or a two pole circuit breaker.

Presently, such multiwire branch circuits are provided with shortcircuit and overcurrent protection by the tied single pole breakers orthe two pole breaker. Only the two pole breaker can also provide groundfault protection by the addition of a common ground fault detector.

Recently, there has been an increased interest in providing protectionfrom arc faults. Arc faults are intermittent high impedance faults whichcan be caused for instance by worn insulation, loose connections, brokenconductors, and the like. Because of their intermittent and highimpedance nature, they do not generate currents of sufficientinstantaneous magnitude or sufficient average current to trigger thethermal-magnetic trip device which provides the short circuit andovercurrent protection. Various types of arc fault detectors have beenproposed, but to my knowledge they have not been adapted to multiwirebranches.

Parent application Ser. No. 08/939,263 filed on Sept. 23, 1998 disclosesapparatus for detecting faults in multiwire branch circuits where bothpoles are tripped simultaneously in response to all faults, overcurrent,arcing and ground faults, regardless of which line conductor isinvolved. This arrangement complies with the general conditions formultiwire branch circuits discussed above where all ungroundedconductors must be opened simultaneously. This is because the two lineconductors can be connected to a single appliance, such as an electricstove for instance, to provide 220 volts, or to two separate outlets ina common receptacle. However, where the two line conductors, with thecommon neutral, are connected to independent loads, both circuits do nothave to be interrupted simultaneously in response to an overcurrentcondition (a short circuit or overload) in one line conductor. By onlyinterrupting current in the affected line conductor, disruption ofservice is minimized. On the other hand, an arcing fault occurs becauseof a wiring failure. This wiring failure may be in the multiwire cable,and therefore, both poles should be tripped.

There is a need for a circuit breaker which can provide arc faultprotection as well as overcurrent, and ground fault protection formultiwire branch circuits and which isolates overcurrent trips to theaffected circuit while tripping both circuits in response to arc faultsor ground faults.

SUMMARY OF THE INVENTION

These needs and others are satisfied by the invention which is directedto an apparatus for detecting faults in multiwire branch circuits. Itincludes a two pole circuit breaker having a first pole connected tointerrupt current in the first line conductor and a second poleconnected to interrupt current in the second line conductor. The firstand second poles have first and second thermal magnetic trip deviceswhich operate independently to only interrupt an overcurrent conditionin the affected pole. The apparatus further includes fault detectioncircuitry including a first arc fault detector connected to detect arccurrents in the first line conductor and to generate a first trip signalin response thereto, a second arc fault detector connected to detect arccurrents in the second line conductor and to generate a second tripsignal in response thereto, and a ground fault detector connected todetect ground faults between each of the line conductors and ground andgenerate a third trip signal. The apparatus also includes meansresponsive to any of these three trip signals to trip both poles of thecircuit breaker. Preferably, the ground fault detector detects neutralto ground faults in addition to line to ground faults. It is alsopreferred that the ground fault detector have a power supply fed by eachof the line conductors so that it remains operative even with one lineunpowered.

Preferably, the arc fault detectors utilize the bimetal of therespective pole which is connected in series with the associated lineconductor as a known impedance for monitoring, the line current. In suchan arrangement, the first and second arc fault detectors are referencedto the associated line conductor, and the trip signals are electricallyisolated such as by optocouplers from the common trip circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the invention can be gained from the followingdescription of the preferred embodiments when read in conjunction withthe accompanying drawings in which:

FIG. 1 is a schematic diagram partly in block form of apparatus inaccordance with the invention.

FIG. 2 is a schematic diagram of the arc fault detectors which form partof the apparatus of FIG. 1.

FIG. 3 is a schematic diagram of the ground fault power supply.

FIG. 4 is a schematic diagram of the power supplies for the arc faultdetectors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a multiwire branch circuit 1 of an electric powerdistribution system includes a first line conductor 3, a second lineconductor 5 and a common neutral conductor 7. As previously mentioned,the two line conductors 3 and 5 are energized by separate phases, or acenter tapped single phase supply voltage. Typically, the neutralconductor is grounded as shown at 9. Protection in this multiwire branchcircuit 1 is provided by a two pole circuit breaker 11 which includesseparable contacts 13₁ and 13₂ connected in series in the lineconductors 3 and 5, respectively. Each pole has a thermal-magnetic tripdevice 14₁ 14₂ which includes a bimetal 15₁ and 15₂ connected in seriesin the associated line conductor. The bimetals 15 respond to the heatgenerated by persistent overcurrent conditions to trip a spring poweredoperating mechanism 16₁, 16₂ which is mechanically connected to open theassociated set of separable contacts 13₁ or 13₂. The thermal-magnetictrip devices also include a magnetic actuator 18₁, 18₂ which actuatesthe associated operating mechanism to instantaneously open thecorresponding contacts 13₁ or 13₂ in response to very high overcurrentssuch as those associated with a short circuit. The thermal-magnetic tripdevices 14₁, 14₂ operate independently, and hence, only interruptovercurrents in the associated pole.

The two pole circuit breaker 11 is located in a load center (not shown)which provides for distribution of power in various circuits such as themultiwire branch circuit 1. In addition to the instantaneous and delayedovercurrent protection provided by the thermal-magnetic trip devices,the two pole circuit breaker also includes arc fault protection for eachof the line conductors 3 and 5 and ground fault protection for all threeconductors 3, 5 and 7. Separate arc fault detectors 17₁ and 17₂ areprovided for the line conductors 3 and 5, respectively. Each of thesearc detectors includes a current sensor to detect current in theassociated line conductor. In the preferred embodiment of the invention,the bimetal 15 is used as the current sensor. As the low resistance ofthe bimetal 15 is known, the voltage drop across the bimetal is ameasure of the current in the associated line conductor. The voltagesacross the bimetals are sensed through the leads 19₁ and 19₂. Since thearc fault detectors 17 use the associated bimetal 15 as a currentsensor, they need to be referenced to the associated line voltage.Accordingly, the Line 1 arc fault detector 17₁ has a ground 21₁referenced to the line conductor 3 while the Line 2 arc fault detector17₂ has a ground 21₂ referenced to the line 2 conductor 5. Each of thearc fault detectors 17₁ and 17₂ has its own power supply 23₁ and 23₂connected between the associated line conductor and the neutralconductor 7.

Ground fault protection is provided by the ground fault detector 25. Inthe preferred embodiment of the invention, the well known dormantoscillator type ground fault detectors employed. A first ground faultdetector coil 27 encircles all three of the conductors 3, 5 and 7. Inthe absence of a ground fault, the resultant current through the coil27, carried by the three conductors will be zero. A ground fault oneither of the line conductors 3 or 5 will create an imbalance in thecurrents which will be detected by the coil 27. As the neutral conductor7 is grounded at 9 close to the circuit breaker 11, a ground faultbetween neutral and ground will not generate a sufficient imbalance incurrent for the coil 27 to detect. A second coil 29 is used to inject asmall voltage into the neutral conductor. If a ground fault is presenton the neutral conductor, a loop completed by this ground fault willsupport an oscillation which will be detected by the ground faultcircuitry.

The ground fault detector 25 is powered by a ground fault power supply31. The ground fault power supply 31 is connected to both of the lineconductors 3 and 5 by the leads 33₁ and 33₂ so that the ground faultdetector 25 is operational if at least one of the line conductors 3 or 5is energized and the contacts 13 of the circuit breaker are closed. Acommon lead 35 is connected between the ground fault power supply 31 andthe neutral conductor 7. As will be seen, with both line conductors 3and 5 energized the output voltage of the ground fault supply providedon the leads 37₁ and can be as high as about 300 volts dc. This voltageis used to energize shunt trip coils 39₁ and 39₂ which results in theopening of both sets of separable contacts 13₁ and 13₂ when a siliconcontrolled rectifier (SCR) 41 is turned on in a manner to be discussed.A circuit 43 draws power from the ground fault power supply 31 throughthe coils 39₁ and 39₂ to energize the ground fault detector circuit 25.The current drawn by this power circuit 43 is insufficient to energizethe coils to open the contacts 13, but is sufficient to operate theground fault detector 25. The circuit 43 includes a zener diode 45 whichclamps the voltage across a capacitor 47 to about 43 volts. A resistor49 forms a filter with the capacitor 47 for this 43 volts dc. Anotherresistor 51 limits the current drawn by the circuit 43 to below thelevel needed to energize the coil 37 and open the separable contacts 13.The 43 volts dc is applied to the chip implementing the ground faultdetector 25 which contains an arrangement of zener diodes represented bythe zener diode 53 which generate 26 volts and other voltages needed bythe ground fault detector circuit 25. This 26 volt dc is filtered by acapacitor 55.

The Line 1 arc fault detector 17₁ generates a trip signal on a lead 57₁in response to detection of an arc fault on the line 1 conductor 3.Similarly, the Line 2 arc fault detectors 17₂ generates a trip signal onthe lead 57₂ in response to detection of an arc fault on the line 2conductor 5. The ground fault detector 25 generates a trip signal on thelead 59 in response to detection of a ground fault between any one ofthe conductors 3, 5 and 7 and ground. The ground faults detected by thedetector 25 can be direct faults to ground Dr themselves can be arcingfaults between a conductor and ground.

A common trip circuit 61 responds to a trip signal generated by any oneof the arc fault detectors 17₁ or 17₂ or the ground fault detector 25.This common trip circuit 61 includes the SCR 41, which when turned onenergizes the trip coils 39₁ and 39₂ to open the separable contacts 13.As the arc fault detectors 17₁ and 17₂ are referenced to the lineconductor on which they are providing protection, the three detectorcircuits must be electrically isolated from one another. This isolationis provided by optocouplers 63₁ and 63₂ which convert the trip signalson the leads 57₁ and 57₂ to signals on leads 65₁ and 65₂ of the commontrip circuit 61. Drive current for the optoisolators 63₁ and 63₂ isprovided from the 26 volt dc supply of the ground fault detector circuit25 through resistor 67. The leads 65₁ and 65₂ from the optoisolators areconnected in parallel which each other and with the lead 59 from theground fault detector 25 to the gate of the SCR 41 so that any one ofthe three signals can trip the contacts 13₁ and 13₂ of both poles open.

As will be noted, the thermal-magnetic trip devices only open thecontacts in the associated pole, while the arc fault circuits and theground fault circuit open both poles. This arrangement is used where theloads connected to the two poles are independent. Under suchcircumstances, overcurrents and especially overloads result fromconditions it the load, and hence, disruption is minimized by onlyinterrupting current to the affected load. On the other hand, arcing andground faults are typically wiring problems which could be occurring inthe home run cable. Therefore, in response to these indications of awiring problem, both circuits are interrupted.

FIG. 2 illustrates a suitable arc fault circuit 17₁. A similar circuitcan be provided for the arc fault circuit 17₂ keeping in mind that eachmust be referenced to the line conductor for which it is providingprotection since the bimetal in the conductor is being used for currentdetection. The voltage across the associated bimetal 15 is provided onthe leads 19₁. The arc fault circuit 17 includes a pulse generator 69, acircuit 71 which provides a time attenuated accumulation of the pulsesgenerated by the pulse generator 69, and an output circuit 73 whichprovides a trip signal on the lead 57 .

The pulse generator 69 includes a high pass filter 75 formed by theseries connected capacitor 77 and resistor 79, followed by a low passfilter 81 formed by the parallel connected capacitor 83 and resistor 85.The high pass filter 75 and low pass filter 81 have a band pass in arange of about 400 to 590 Hz which generates pulses in response to thestep increases in current caused by striking of an arc.

An operational amplifier (op amp) 87 provides gain for the pulses. Acapacitor 88 reduces high frequency noise in the pulse signals. The opamp 87 is biased at its non-inverting input by a 13 vdc supply voltage.A resistor 89 and capacitor 91 delay application of the bias to preventfalse trip signals during power up. The positive and negative pulsesgenerated by the band pass filter ride on the plus 13vdc volt biasapplied to the op amp 87. This bias is removed by the ac couplingcapacitor 93 which along with the resistor 95 forms another high passfilter stage. The bi-polar pulse signal resulting is rectified by arectifier circuit 97 which includes another op amp 99. Positive pulsesare applied to the non-inverting input of the op amp 99 through thediode 101 while negative pulses are applied to the inverting inputthrough the diode 103. The output of the op amp 99 is a pulse signalhaving pulses of a single polarity.

The circuit 71 generates a time attenuated accumulation of the pulses inthe pulse signal generated by the pulse generator 69. The pulses areaccumulated on a capacitor 105 connected to the 26 vdc supply. A bleedresistor 107 connected across the capacitor 105 provides the timeattenuation. The pulses are applied to the capacitor 105 through atransistor 109. When no pulses are generated, both electrodes of thecapacitor 105 are at 26 volts. The pulses from the pulse generator 69provide base drive current for the transistor 109. A voltage dividerformed by the resistor 111 and 113 connected at their common connectionto the emitter of the transistor 109 set the minimum amplitude for thepulses to turn on the transistor 109. This threshold is selected so thatpulses which could be generated by some normal loads, such as forinstance a dimmer switch operating at normal loads, are not accumulated.The amplitude of the pulses is set by the gain of the op amp 99 which inturn is determined by the ratio of the feed back resistor 115 and inputresistor 117. The amplitude and duration of each pulse determine theamount of charge which is applied to the capacitor 105. The successivepulses are accumulated through the summation of the charge they add tothe capacitor 105. The resistor 107 continuously bleeds the charge onthe capacitor 105 with a time constant determined by the values of thecapacitor 105 and resistor 107 to time attenuate the accumulation of thepulses. It can be appreciated that the magnitude and time intervalbetween pulses determines the instantaneous voltage that appears acrossthe capacitor 105.

The output circuit 73 monitors the voltage across the capacitor 105representing the time attenuated accumulation of the pulses in the pulsesignal generated by the pulse generator. Each pulse lowers the voltageon the capacitor which is applied to the base of a transistor 119 in theoutput circuit. A voltage is applied to the emitter of the transistor119 by the 13 vdc supply through a resistor 121 and diode 123. With nopulses being generated, the voltage on the base of the transistor 119 is26 volts. Without the diode 123, the 13 volt reverse bias would destroythe base to emitter junction of the transistor 119. The diode 123withstands this voltage. When the voltage at the lower end of thecapacitor 105, and therefore on the base of the transistor 119, fallsbelow the 13 volts minus the forward drop across the diode 123, thetransistor 119 is turned on. Feedback provided through the lead 125 andthe resistors 127 and 129 holds the transistor 119 on by providing acontinuous output of the op amp 99 which holds the transistor 109 on.Turn on of the transistor 119 provides base drive current for thetransistor 131 which draws current limited by the resistor 133 togenerate an arc fault trip signal on the lead 57₁. The trip signalactuates the optocoupler 63₁ which turns on the SCR 41 to trip theseparable contacts 13 open. The larger the pulses in the pulse signalgenerated by the pulse generator 69 the harder the transistor 109 isturned on, and hence, the faster charge is accumulated on the capacitor105.

A circuit diagram of the ground fault power supply 31 is shown in FIG.3. This power supply includes a bridge circuit 135 having six diodes137. Power is supplied to the bridge from the Line 1 conductor 3 throughthe lead 33₁ and from the Line 2 conductor 5 through the lead 33₂.Output of the bridge is between the leads 37₁ and 37₂ and the groundfault common 139. The lead 33₁ is connected to the mid-point of one legof the bridge 135 while the lead 33₂ is connected to the mid-point ofanother leg. The neutral conductor 7 is connected to the mid-point ofthe third leg through the lead 35. A pair of metal oxide varistors(MOVs) 141 protect the power supply 31 from voltage surges on the lineconductors. With both line conductors energized the output of the powersupply 31 across the lead 37 and common 139 is the line to line voltage.With only one line conductor energized, the output of the power supply31 is the line to neutral voltage. As can be seen, the potential of theground fault common 139 changes. When the diode 137₁ is conducting,ground fault common 139 is tied to the voltage on Line 1. With the diode137₃ conducting, it is tied to the voltage on Line 2, and if one line isnot energized so that diode 137₅ conducts on positive half cycles of theline voltage, the ground fault common 139 is tied to neutral.

FIG. 4 illustrates the power supply 231 for the Line 1 arc faultdetector 17₁. This power supply is connected on the hot side to theneutral conductor 7 through the lead 143. The other side of the powersupply is connected to the Line 1 conductor 3 through the Line 1 arcfault detector common 21₁. The diode 145 halfwave rectifies the neutralto Line 1 voltage and the resistor 147 converts this rectified voltagesignal to about a 6 milliamp current which charges a capacitor 149. Thevoltage across capacitor 149 is clamped to 43 volts dc by the zenerdiode 151. Resistor 153 and capacitor 155 form a second filter. Thevoltage across the capacitor 155 is clamped at 26 volts dc by the zenerdiodes 157 and 159 to provide the 26 vdc₁ for the Line 1 arc faultdetector 17₁. The common junction between zener diodes 157 and 159provides the 13 vdc₁ supply voltage for arc fault detector 17₁. Thepower supply 23₂ for the Line 2 arc fault detector 17₂ has a similarcircuit configuration except that it is referenced to the common 21₂.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular arrangement disclosed are meant to be illustrative only andnot limiting as to the scope of invention which is to be given the fullbreath of the claims appended and any and all equivalents thereof.

What is claimed is:
 1. Apparatus for detecting faults in multi-wirebranch circuits including a first line conductor and a second lineconductor fed by separate phases or a single center tapped phase, and acommon neutral conductor, said apparatus comprising:a two-pole circuitbreaker having a first pole with a thermal-magnetic trip deviceconnected to interrupt current in said first line conductor in responseto an overcurrent, condition in said first line conductor, and a secondpole with a second thermal-magnetic trip device connected to interruptcurrent in said second line conductor in response to an overcurrentcondition in said second line conductor, said first and second magnetictrip devices operating separately to only interrupt current in the firstand second line conductors, respectively; and fault detection circuitryincluding a first arc fault detector connected to detect arc currents insaid first line conductor and generate a first trip signal in responsethereto, a second arc fault detector connected to detect arc currents insaid second line conductor and generate a second trip signal in responsethereto, a ground fault detector connected to detect ground faults ineach of said first line conductor and said second line conductor andgenerate a third trip signal in response thereto, and response meansresponsive to any of said first trip signal, said second trip signal andsaid third trip signal to trip both said first pole and said second poleof said two-pole circuit breaker open simultaneously, said responsemeans comprising a common trip circuit and means electrically isolatingsaid first trip signal, said second trip signal and said third tripsignal from each other.
 2. The apparatus of claim 1 wherein said groundfault detector further includes means detecting neutral to groundfaults.
 3. The apparatus of claim 2 wherein said ground fault detectorincludes a ground fault power supply supplied by each of said first lineconductor and second line conductor.
 4. The apparatus of claim 3 whereinsaid first pole includes a first known impedance in series in said firstline conductor, said first arc fault detector includes means monitoringa first voltage across said first known impedance as an indication ofcurrent in said first line conductor, and means referencing said firstvoltage to said first conductor, and wherein said second pole includes asecond known impedance connected in series with said second lineconductor, said second arc fault detector includes means monitoring asecond voltage across said second known impedance as an indication ofcurrent in said second line conductor, and means referencing said secondvoltage to said second line conductor.
 5. The apparatus of claim 4wherein said first arc fault detector further includes a first powersupply referenced to said first line conductor, and wherein said secondarc fault detector includes a second power supply reference to saidsecond line conductor.
 6. The apparatus of claim 4 wherein said firstknown impedance is a first bimetal in said first thermal-magnetic tripdevice, and wherein said second known impedance is a second bimetal insaid second thermal-magnetic trip device.
 7. Apparatus for detectingfaults in multi-wire branch circuits including a first line conductorand a second line conductor fed by separate phases or a single centertapped phase, and a common neutral conductor, said apparatuscomprising:a two-pole circuit breaker having a first pole with the firstthermal-magnetic trip device connected to interrupt current in saidfirst line conductor in response to an overcurrent condition in saidfirst line conductor, a first known impedance in series with said firstline conductor, and a second pole with a second thermal-magnetic tripdevice connected to interrupt current in said second line conductor inresponse to an overcurrent condition in said second line conductor and asecond known impedance connected in series with said second lineconductor, said first and second magnetic trip devices operatingseparately to only interrupt current in the first and second lineconductors, respectively; and fault detection circuitry including:afirst arc fault detector connected to detect arc currents in said firstline conductor and generate a first trip signal in response thereto andincluding means monitoring a first voltage across said first knownimpedance as a measure of current in said first line conductor and meansreferencing said first voltage to said first conductor; a second arcfault detector connected to detect arc currents in said second lineconductor and generate a second trip signal in response thereto andincluding means monitoring a second voltage across said second knownimpedance as an indication of current in said second line conductor, andmeans referencing said second voltage to said second line conductor; aground fault detector connected to detect line to ground faults in eachof said first line conductor and second line conductor and generating athird trip signal in response thereto; and means responsive to any ofsaid first trip signal, said second trip signal and said third tripsignal to trip said first pole and said second pole of said circuitbreaker open simultaneously.
 8. The apparatus of claim 7 wherein saidfirst known impedance is a first bimetal in said first thermal-magnetictrip device and, and wherein said second known impedance is a secondbimetal in said second thermal-magnetic trip device.
 9. The apparatus ofclaim 8 wherein said first arc fault detector further includes a firstpower supply referenced to said first line conductor, and wherein saidsecond arc fault detector includes a second power supply referenced tosaid second line conductor.
 10. The apparatus of claim 7 wherein saidground fault detector also includes means detecting a neutral to groundfault and generating a said third signal in response thereto, andwherein said ground fault detector is powered by a ground fault powersupply supplied by each of said first line conductor and second lineconductor.